It is known to provide a motorised reduction gear unit comprising an electric motor, the rotor shaft of which is extended so as to carry or incorporate a worm which penetrates into a hollow casing which contains a speed reduction mechanism, the casing being closed by a cover plate. The speed reduction mechanism, or system, comprises a toothed wheel which is driven in rotation by the worm and which acts, through an eccentric axis or gudgeon pin carried by the toothed wheel, a crank and connecting rod system. The crank and connecting rod system converts the continuous rotary motion of the toothed wheel (and therefore of the motor) into alternating rotary motion of an output shaft, which is carried by the casing and which passes through the base of the casing for connection to the remainder of the screen wiping apparatus. In this connection, the output shaft is connected conventionally to at least one screen wiper consisting of a screen wiper arm and a wiper blade which wipes over a predetermined angular sector of the windshield of the vehicle.
Because of the development of windshields which have become more and more panoramic in design, and which have also become inclined more and more with respect to the vertical, the size of the associated screen wipers has to be increased. A consequence of this is that they are also becoming heavier. In addition, the wiping angle traversed by the screen wipers is becoming greater and greater, and may reach 180 degrees.
This leads to various problems in connection with the drive system which is provided for the screen wiper. In this connection, as has been seen above, the output shaft is driven by a crank and connecting rod system in which the connecting rod consists of an arm which is connected at one end to the eccentric gudgeon pin carried by the toothed wheel; while, at its other end, the arm has a toothed sector which meshes with another toothed sector, this other toothed sector being coupled fixedly to the output shaft.
While the windshield wiper is moving over a zone going from its parking position to a position at 90 degrees from the parking position, the two toothed sectors of the drive system cooperate fully with each other, and the wiping motion is regular. By contrast, once the wiper has passed beyond this zone, and is traversing an angular sector which is more than 90 degrees away from its parking position, the windshield wiper now has sufficient stored inertia for the wiper itself to become a motor element with respect to the motorised reduction gear unit. It thus happens that the toothed sectors are no longer fully in mesh. The teeth of the toothed sector which is connected to the output shaft are therefore able to escape from the teeth of the other toothed sector, i.e. the one which is situated at the end of the arm driven by the toothed wheel. When this happens, the screen wiper is no longer in controlled movement as it passes over the zone concerned, and it may no longer be being driven by the electric motor.
In addition, when the screen wiper changes direction so as to start back towards its parking position, a sudden jolt or shock is produced at the instant when the teeth of the two toothed sectors meet. This gives rise to noises and also to irregularities in the wiping action, which can be disagreeable to the occupants of the vehicle.
Solutions have been proposed for the problem of reducing these shocks in the extreme wiping positions. For example, U.S. Pat. No. 5,182,957 and European published patent specification EP 0 498 949A describe end stops based on the following principle, to act as abutments at the end of travel of the wiper.
An annular stop member surrounds the output shaft and is coupled to the latter for rotation with it. It is provided with a lug which slides in a groove surrounding the output shaft. This groove has at both its ends, corresponding to the extreme wiping positions, resilient elements into which the lug of the stop member enters when the screen wiper reaches its extreme positions. The effect of this is to reduce shocks in the screen wiper and the resulting noise. However, this does not resolve the problem of acceleration of the screen wiper when it passes beyond the wiping position at 90 degrees away from its parking position. In addition, that solution involves adding to the reduction gear apparatus a plurality of components of complex design, which complicates manufacture of the unit.